Trapping Waves in Earth's Plasmasphere

Earth’s magnetic field traps donut-shaped bands of radiation in a belt around the planet that react to solar eruptions by growing and shrinking. The Van Allen belts consist of two rings filled with particles from the solar wind and cosmic rays. Within the outer ring of the Van Allen belt sits the plasmasphere, which is the innermost part of the planet’s magnetic field and home to low-energy charged particles.

The planet’s magnetic field is crucial for life below, and solar storms in the region can wreak havoc on satellites and technologies necessary for modern life.

In 2012, NASA launched two identical spacecraft called the Van Allen Probes to study the little-­explored region. Ma et al. used data from these spacecraft to study how magnetosonic waves—waves of ions traveling perpendicular to the magnetic field—propagate, excite, and are trapped by Earth’s plasmasphere. The waves are observed at our planet’s magnetic equator, where the field seen on Earth’s surface is horizontal instead of vertical, as it is at the poles.

The researchers specifically examine an outer region of Earth’s plasmasphere called the plasmapause, where the density of particles drops off dramatically.

The team analyzed magnetosonic wave events on 6 November and 4 December 2012. They found that the magnetosonic waves were excited near the plasmapause and propagated into the plasmasphere, where they then became trapped. The authors say the study explains how magnetosonic waves can be found in regions far from where they are created. (Geophysical Research Letters, doi:10.1002/2014GL061414, 2014)

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